Data converters are provided for receiving either an analog signal for conversion to a digital signal or a digital signal for conversion to analog signal. For conversion of analog signals to digital signals, an analog-to-digital converter is utilized. This is typically facilitated by sampling an analog voltage onto a capacitor array having a plurality of binary weighted capacitors. The capacitors then have the ability to have one plate thereof selectively switched between a reference voltage and ground to redistribute the charge among the capacitors, the switching done in a sequential manner in accordance with a successive approximation algorithm. By selectively switching the plates of the capacitors, and comparing the other plate of the capacitors, which is connected to a common input of a comparator, to a reference voltage, a digital value for the analog voltage sampled at the input can be determined.
A number of problems exist with the data conversion of an analog signal to a digital signal. Some of these problems reside in the various offsets of the inputs to the comparators, one of which is due to the fact that the actual chip ground may be different from the input ground at the PC board on which the actual chip is disposed. Additionally, the capacitors in the capacitor array are weighted and can have errors associated therewith. These errors can be accounted for by actually calibrating each of the capacitors with a sub-capacitor array. However, this calibration must be done at each power up of the A/D convertor. Additionally, these capacitor arrays can also have various parasitics associated therewith that effect the operation thereof and require the driving voltage to drive a higher capacitance value than that associated with the capacitance array.
When the capacitor arrays are operated in accordance with a data conversion algorithm such as a SAR algorithm, during the sampling period, the output node of the capacitor array is typically connected to an input of an amplifier and that input connected to a reference voltage. When operating in conjunction with a differential input amplifier, typically both input nodes thereof are switched to a common mode voltage during the sampling or tracking phase where the input voltage is impressed across the switched capacitors and then switched to the capacitor array thereafter. However, it is important when operating with a single array that noise introduction by the voltage source driving the common mode node or reference node for each of the inputs is cancelled. Unless these are balanced, there will be a noise contribution due to this reference voltage circuit.
During the operation of the data conversion, the capacitors disposed in the capacitor array will have various voltages switched thereto. One of these voltages is a common-mode reference voltage. Typically, this reference voltage is generated from an external supply to the array, this typically being generated on-chip, and is a high impedance driving signal. Therefore, it is not capable of driving a low impedance circuit. To facilitate driving the capacitor array to the common-mode reference voltage level, a high-current driver will be provided in the form of an amplifier. These are typically operated with a reference voltage at the common mode level. However, the operation of the amplifier can be affected by the capacitive loading of the common mode amplifier. During the switching operation, the capacitors will be switched such that there is a charge redistribution across capacitors in the array, with a resulting transient at the beginning of each switch operation. After each switching operation, there is a compare operation where the voltage on the capacitor array is compared to a reference voltage. This compare operation need to be long enough to allow the transients to settle, which settling tome is directly related to the sampling rate of the SAR convertor.